remount: Remove fs_mgr_overlayfs_scratch_device()

This is no longer needed now that GetScratchDevice exists. The cache can
go away too, since it only existed to avoid libdm spam. The spam is
avoided by checking GetState before calling GetDmDevicePathByName.

Bug: 134949511
Test: adb remount and adb_remount_test.sh
Change-Id: I9d94e2523af9de394a811d0b398fe20415708a6b